In devices of the kind to which the invention applies, in particular for sensing fingerprints, the source of radiation (generally visible light) illuminates the body part (e.g. the finger); the portions of the part in contact with the face diffuse the radiation, and the diffused radiation passes through an optical system in order to each the sensitive surface of a sensor. That type of device operates well so long as the face on which the body part is placed is itself clean.
Nevertheless, after a body part such as a finger has been placed thereon, it is common for traces of grease to remain on the face. When the only radiation reaching said face comes from the source within the device, such traces of grease diffuse relatively little radiation, and the optical system is arranged to ensure that the sensor is not excited.
In contrast, it can happen that radiation from a relatively powerful external source (the sun, a halogen lamp, . . . ) reaches the above-mentioned face at an angle of incidence. Under such circumstances, the traces of grease present on the face can diffuse a relatively large quantity of light, that then reaches the sensor and excites it. The sensor then delivers an image of the traces of grease (a latent image). If these traces of grease are sufficient, the latent image as obtained in this way can reproduce sufficiently clearly a biometric characteristic (e.g. a fingerprint) of a person who has been processed previously, and that can lead to a wrong command (wrong detection that a body part has been applied, wrong authorization, . . . ).
Various solutions are known that have been proposed in order to attempt to solve the problem posed and to attempt to prevent the device from operating in response to latent images.
Thus, document US 2003/0202687 proposes forming a plurality of images of the body part which are then compared with one another. A difference detected between the images indicates that the part has moved, and makes it likely that a genuine body part is present, whereas strict identity between the images is representative of no movement and makes it likely that the images are latent images. A drawback of that prior art solutions lies in the fact that if the external source of radiation moves or if the system for acquiring biometric characteristics itself moves, then the latent images can vary in time and can be processed as though a body part was in place against the face.
Document WO 01/84477 relates to a device having a controllable internal radiation source. If images are acquired both with radiation and without radiation, then it becomes possible to discriminate between latent images and images of a body part for processing. Nevertheless, that prior art device presents the drawback of being ineffective when the external source switches off at the same time that the internal source, and it can also operate wrongly when the external source is not constant (e.g. sunlight through foliage, busy premises, moving sensor).
Document WO 97/14111 teaches the use of electrodes for measuring electrical properties at the surface of the optical element in order to detect the presence of a body part. Such a device which is intended in practice to detect fraud requires a specific analog electronic circuit; it is complex and expensive.